Means of Introducing an Analyte into Liquid Sampling Atmospheric Pressure Glow Discharge

ABSTRACT

A liquid sampling, atmospheric pressure, glow discharge (LS-APGD) device as well as systems that incorporate the device and methods for using the device and systems are described. The LS-APGD includes a hollow capillary for delivering an electrolyte solution to a glow discharge space. The device also includes a counter electrode in the form of a second hollow capillary that can deliver the analyte into the glow discharge space. A voltage across the electrolyte solution and the counter electrode creates the microplasma within the glow discharge space that interacts with the analyte to move it to a higher energy state (vaporization, excitation, and/or ionization of the analyte).

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation application of U.S. patentapplication Ser. No. 14/171,981 having a filing date of Feb. 4, 2014,which claims filing benefit of U.S. Provisional Patent Application Ser.No. 61/760,824 having a filing date of Feb. 5, 2013, which isincorporated herein in its entirety.

GOVERNMENT SUPPORT CLAUSE

This invention was made with government support under grant#DE-AC02-05CH11231 awarded by the Department of Energy and grant#DE-AC06-76RL01830 awarded by the Department of Energy. The governmenthas certain rights in the invention.

BACKGROUND

The inductively coupled plasma (ICP) has been a standardexcitation/ionization source for the analytical techniques of opticalemission spectroscopy (OES) and elemental mass spectrometry (MS) formany years.

Unfortunately, the general trend toward more compact and portablespectrochemical sources has been slower to reach elemental (atomic)spectrometry than other fields, largely because the ICP source hasremained the workhorse of the field. In addition to their size, ICPsources typically consume large amounts of gas and sample solution.

The liquid sampling, atmospheric pressure, glow discharge (LS-APGD) is alow power, small footprint source that has been used in analyticaltechniques such as optical emission spectroscopy and mass spectrometry(see, e.g., U.S. Pat. Nos. 6,852,969, 6,750,449, 5,325,021, 5,086,226,and 5,006,706, all of which are incorporated herein by reference). Themicroplasma of these methods operates at power densities of about 10W/mm³, much higher than the typically-cited value of about 0.1 W/mm³ forICP. LS-APGD was originally developed for applications in metalspeciation, being operable at low solution flow rates (<400 μL/min) andemploying an electrolytic solution (5% acid/salt) as the mobile phase.In the direct solution analysis mode, heat is generated as current flowsacross the air/liquid interface and causes evaporation, eventuallyculminating in excitation of the analyte species passing through themicroplasma. For quantitative analysis, detection limits for aqueoussamples are at the single nanogram level (using relatively simpleoptical spectrometer systems). The microplasma environment (e.g.,kinetic and excitation temperatures) is more in line with combustionflames than other atmospheric pressure plasmas (e.g. ICPs). Therobustness of the microplasma with regard to changes in solutionmatrices is similar to ICP sources. The use of LS-APGD has positiveattributes in terms of design simplicity, small footprint, low operatingpowers, and very low liquid flow rates resulting in no liquid waste.Building on the development of the low power microplasma, the LS-APGDsource has brought compactness and low gas and sample consumption to theelemental analysis of flowing samples.

Laser ablation has become a prominent technology for sample introductionin direct chemical analysis of solids, particularly in the case ofmicroanalysis. For analytical purposes, laser ablation has been commonlybased on two primary measurement modalities: laser-induced breakdownspectroscopy (LIBS) and laser ablation-inductively coupled plasma-massspectrometry (LA-ICP-MS). LIBS-based analyses provide unique advantagesthat include rapid in situ simultaneous multi-element analysis.LA-ICP-MS-based analyses provide enhanced sensitivity and theopportunity of isotopic analysis over LIBS, but require more expensivelaboratory-based instrumentation. One of the main advantages of LIBS isin situ analysis—no secondary excitation source. This capability isavailable because the sampling, vaporization, and excitation processesare complete in one step, while in the case of LA-ICP-MS, the samplingand vaporization/excitation/ionization processes are completed inseparate (in time and space) steps; consequently they may beindividually optimized.

In both technologies, a focused laser beam converts a small portion of asolid sample into an aerosol. For LA-ICP-MS, the ideal analyte of theaerosol is comprised of small, uniform-sized particles that can beentrained and transported efficiently to the ICP. For LIBS, the idealanalyte of the aerosol is vapor that is excited to atomic and ionicoptical emission, as particulates do not contribute to the measurement.Fundamentally, both aerosol forms can be produced, as well as aerosolscarrying liquid phase analyte, with the composition established by theexperimental (ablation) parameters.

For LIBS, the most successful way (to date) to improve sensitivity hasbeen by increasing excitation efficiency in the laser-induced plasmawith the use of a second laser pulse (double pulse, DP) delayed from thesampling pulse. The enhancement in spectral line emission intensityusing DP excitation depends on several parameters, including inter-pulsedelay time, plasma density, laser wavelength, and line excitationenergy. The enhancement is proposed to be due to higher plasmatemperature and/or larger and longer plasma duration, as well asincreased ablated mass (particularly in the case of collinearconfigurations). Extension to the use of the ICP source to enhance theanalyte excitation/ionization is well characterized and implementedacross a diverse range of applications. However, there still existquestions as to the practicality of using a 1-2.5 kW rf plasma, andhaving a sampling volume of about 1 cm³ to analyze sample mass on theorder of nanograms and below.

Overall, laser ablation is well established as an excellent method fordirect solid sampling and introduction of the sample into conventionalspectroscopic sources such as the ICP. The method allows real-timeanalysis without sample preparation and requires a significantly lowerquantity of mass than conventional sample dissolution procedures.Research over the years has addressed laser wavelength and pulseduration as critical parameters defining accuracy, sensitivity andprecision. The femtosecond pulsed ablation process has been shown toproduce a narrow, nanometer-sized, particle distribution that is idealfor consumption in the ICP. However, as the spatial resolution isimproved and the quantity of sampled mass is significantly reduced, theconventional ICP torch becomes a rather large diluting source that isnot necessary for the digestion of femtograms or less material.

What is needed in the art are secondary sources for vaporization,excitation, and/or ionization of an analyte that are of physicaldimensions that are complementary to the small sample sizes such as arepresent in laser ablation samples. It would be highly beneficial if suchsources were available at much lower operational costs, smallerfootprint, lower energy consumption, and with a practical cost tobenefit ratio.

SUMMARY

According to one embodiment, a liquid sampling, atmospheric pressure,glow discharge device is disclosed. For instance, the device can includea first hollow capillary having a discharge end and a counter electrodethat is disposed at a distance from the discharge end of the firsthollow capillary. The counter electrode is the terminal portion of asecond hollow capillary. The device also includes a power source that isin electrical communication with a conductive element in the firsthollow capillary and also in communication with the counter electrode.The power source is configured to maintain a glow discharge in a glowdischarge space. The glow discharge space comprises a space in which aflow discharged from the first hollow capillary intersects a flowdischarged from the second hollow capillary.

A system is also disclosed that includes a liquid sampling, atmosphericpressure, glow discharge device and a laser ablation device. The laserablation device can be in fluid communication with the glow dischargedevice such that an aerosol carrying an analyte can flow from the laserablation device and through the second hollow capillary to be dischargedinto the glow discharge space of the glow discharge device. During use,the glow discharge space can comprise a microplasma. The system can alsoinclude an instrument for analyzing the analyte of the aerosol followinginteraction of the analyte with the microplasma. The instrument can be,for instance, a monochromator or a mass spectrometer.

In another embodiment a method for examining an analyte is disclosed.The method can include flowing an electrolyte solution through a firsthollow capillary of a liquid sampling, atmospheric pressure, glowdischarge device such that the electrolyte solution is discharged from adischarge end of the first hollow capillary. The first hollow capillarycan include an electrically conductive element that is upstream of or atthe discharge end of the first hollow capillary and in electricalcommunication with the electrolyte solution that flows within the firsthollow capillary. The method also includes flowing an aerosol thatcontains an analyte through a second hollow capillary such that theanalyte is discharged from a discharge end of the second hollowcapillary. The discharge end of the second hollow capillary being acounter electrode. The method further includes connecting a power sourcebetween the electrically conductive element and the counter electrode soas to maintain a glow discharge in a glow discharge space, the glowdischarge space comprising a space in which flow discharged from thefirst hollow capillary and flow discharged from the second hollowcapillary intersect. When the aerosol is discharged from the secondhollow capillary and into the glow discharge space, the analyte of theaerosol can interact with a microplasma formed within the glow dischargespace and move to a higher energy state. For instance, the interactionbetween the microplasma and the analyte can lead to the analyte beingvaporized, excited, and or ionized by the microplasma. Following thisinteraction, the analyte can be examined as desired.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure may be better understood with reference to thefollowing figures in which:

FIG. 1 is a diagrammatic representation of the operation of a prior artembodiment of an LS-APG apparatus.

FIG. 2 is a diagrammatic representation of a prior art LS-APGD apparatusoperating in an optical emission mode.

FIG. 3 is a diagrammatic representation of an LS-APGD as describedherein.

FIG. 4 is an expanded view of a portion taken from FIG. 3.

FIG. 5A and FIG. 5B schematically present from a cross-sectional view,alternative relative orientations of the counter-electrode to thedischarge end of the first hollow capillary of an LS-APGD.

FIG. 6 schematically illustrates from a cross-sectional view anotherorientation of the counter-electrode to the discharge end of the firsthollow capillary.

FIG. 7 schematically illustrates one embodiment of a laser ablationdevice as may be utilized in conjunction with an LS-APGD device asdisclosed herein.

FIG. 8 schematically illustrates one embodiment of an LA-LS-APGD systemas described herein operating in an optical emission mode.

FIG. 9 compares the ion intensities produced by the femtosecond laserablation of a copper shard, m/z=62.93, and a common hydrocarbonbackground contaminant C₄H₇, m/z=55.06.

FIG. 10 illustrates the depth-profile of a U.S. one cent coin using 110femtosecond laser pulses at 100 Hz. Cu and Zn onset are nearlysimultaneous owing to the high repetition rate of the laser. Backgroundcontaminant ion, C₄H₇, is also shown.

FIG. 11 illustrates the major isotopes of Pb (FIG. 11A) and Cu (FIG.11B) generated by the laser ablation of commercial solder and ionizedwith the LS-APGD ionization source.

FIG. 12 illustrates the expanded mass range spectrum of solder showingits major constituents, Sn and Pb, with associated hydroxide, water andaluminum clusters.

FIG. 13 illustrates the influence of the helium carrier gas flow rate(0.14-0.63 L min−1) on ablated Cu (I) response from an LS-APGDmicroplasma. Background response 1.2×10³. All other parameters constant:discharge current=60 mA, solution flow rate=10 μL min⁻¹ (5% HNO₃),sheath gas=0.3 L min⁻¹ He, 1064 nm laser=55 mJ (rep rate=5 Hz).

FIG. 14 illustrates the Cu (I) emission response as a function of theAPGD source current. Background response 0.6×10³. All other parametersconstant: particle carrier gas=0.3 L min⁻¹, solution flow rate=10 μLmin⁻¹ (5% HNO₃), sheath gas=0.3 L min⁻¹ He, 1064 nm laser=55 mJ (reprate=5 Hz).

FIG. 15 illustrates Cu (I) emission response as a function of laserpower. Background response 1.2×10³. All other parameters constant:discharge current=60 mA, particle carrier gas=0.3 L min⁻¹, solution flowrate=10 μL min⁻¹ (5% HNO₃), sheath gas=0.3 L min⁻¹ He, 1064 nm laser reprate=5 Hz.

FIG. 16 illustrates the laser ablated LS-APGD optical emission spectrameasured from a copper sample from 320 nm to 340 nm (FIG. 16A) and from506 nm to 526 nm (FIG. 16B). Experimental conditions: dischargecurrent=60 mA, particle carrier gas=0.3 L min⁻¹, solution flow rate=10μL min⁻¹ (5% HNO₃), sheath gas=0.3 L min⁻¹ He, 1064 nm laser=44.3 mJ(rep rate=5 Hz).

FIG. 17 illustrates the laser ablated LS-APGD optical emission spectrummeasured from a brass alloy. Working parameters same as listed in FIG.16.

FIG. 18 illustrates laser ablated LS-APGD optical emission signaltransients of Cu (I) (324.75 and 327.39 nm) and Zn (I) (330.25 nm) forparticles ablated from a brass alloy. Working parameters same as listedin FIG. 16.

FIG. 19 illustrates the LIBS optical emission spectra measured for abrass alloy.

FIG. 20 illustrates the Cu (I) 324.75 nm and Zn (I) 330.25 nm responseswith respect to the known percentage of each element across a brassstandard alloy series using LS-APGD and LIBS optical emissionspectrometry.

FIG. 21 illustrates Zn (I)/Cu (I) optical emission ratios (obtained fromFIG. 20) as a function of elemental concentration for a set of brassstandards using LS-APGD and LIBS optical emission spectrometry.

The same reference characters are assigned to the same componentsthroughout the drawings and description.

DETAILED DESCRIPTION

Reference now will be made in detail to embodiments of the disclosedsubject matter, one or more examples of which are illustrated in theaccompanying drawings. Each embodiment is provided by way of explanationof the subject matter, not limitation of the subject matter. In fact, itwill be apparent to those skilled in the art that various modificationsand variations can be made in the present disclosure without departingfrom the scope or spirit of the subject matter. For instance, featuresillustrated or described as part of one embodiment can be used onanother embodiment to yield a still further embodiment. Thus, it isintended that the present disclosure cover such modifications andvariations as come within the scope of the appended claims and theirequivalents.

In general, disclosed herein is a liquid sampling, atmospheric pressure,glow discharge (LS-APGD) device as well as systems that incorporate thedevice and methods for using the device and systems. More specifically,the LS-APGD includes a hollow capillary for delivering an electrolytesolution to a glow discharge space. The device also includes a counterelectrode in the form of a second hollow capillary that can deliver anaerosol containing the analyte into the glow discharge space. A voltageacross the electrolyte solution and the counter electrode creates themicroplasma within the glow discharge space. The analyte of the aerosolcan then be acted upon by the microplasma. Depending upon the nature ofthe analyte, the microplasma can vaporize, excite, and/or ionize theanalyte. For instance, an analyte that is in the liquid or solid phasecan first be vaporized by the microplasma followed by excitation andoptionally also ionization, depending upon the nature of the analyte andthe microplasma. An analyte that is already in a gaseous state can beexcited and/or ionized by the microplasma.

The disclosed LS-APGD device can be similar to previously known devices,but incorporate the hollow capillary counter electrode. The LS-APGDdevices can be particularly useful when analyzing aerosols containingparticulate analytes, and in one embodiment, laser ablated particulates.However, it should be understood that the analyte of the aerosol canalternatively be in the gas phase or the liquid phase. In oneembodiment, the device can be utilized in conjunction with a laserablation device, for instance in an analysis system that can alsoinclude one or more instruments such as a polychromator, amonochromator, and/or a mass spectrometer for analyzing ions formed inthe microplasma.

The disclosed devices can facilitate the direct introduction of theanalyte into the microplasma as well as the use of the low powermicroplasma for exciting/ionizing the analyte. This can be especiallybeneficial when considering analyte samples of a small volume, such asmay be formed in a laser ablation process. Moreover, it has been foundthat the glow discharge microplasma has sufficient thermal energy andelectron number density such that extremely small analyte particles canbe analyzed. For example, nano-sized particles on a single nanometerscale (e.g., less than about 10 nanometers) as may be formed fromfemtosecond pulsed laser ablation process can be ionized by use of theLS-APGD device. In addition, combining a particulate analyte source suchas a laser ablation device and the LS-APGD device with an analysisdevice such as a monochromator and/or a mass spectrometer can bestraight forward.

FIG. 1 and FIG. 2 generally describe previously known LS-APGD devices.As can be seen with reference to FIG. 1 and FIG. 2, an LS-APGD devicecan include a power supply 40. The devices also include a capillary 22through which an electrolyte solution 27 can flow. In these devices, theanalyte to be excited/ionized is contained within the electrolytesolution 27. For instance, as shown in FIG. 2, a device can include apump 30, an injector 44, and a fitting 28 connecting electricallyinsulative and an electrically conductive portions of the capillary 22to provide the analyte within the electrolyte solution 27 that passesthrough the capillary 22 and to the discharge end 24 of the capillary22. As shown, the capillary 22 defines a longitudinal axis 26 alignedparallel to the direction of the flow of the electrolyte solution 27.The discharge end 24 is a free end that terminates in a plane that isgenerally perpendicular to the axis 26 of fluid flow upon exiting of thedischarge end 24 of the capillary 22.

The devices of FIG. 1 and FIG. 2 include a solid counter-electrode 34that is axially aligned with the direction 26 of fluid flow upon exitingof the discharge end 24 of the capillary 22. The counter-electrode 34 isdisposed at a predetermined distance from the discharge end 24 of thecapillary 22 forming a glow discharge space 35 in which the glowdischarge 36 containing the microplasma is formed. The power supply 40is in electrical contact with a conductive element 25 (for instance anelectrically conductive capillary section) in the hollow capillary 22that is in turn in electrical communication with the electrolytesolution 27 within the capillary 22. Thus, the electrolyte solution 27and the counter-electrode 34 form the input and output electrodes of theLS-APGD apparatus. The analyte species in the higher energy stateinduced by the microplasma of the glow discharge 36 can be analyzed byuse of a suitable analysis device, such as a light directing element 46in communication with a monochromator 50, as shown.

The disclosed LS-APGD devices differ from previously known devices suchas those illustrated in FIG. 1 and FIG. 2 in that an analyte isdelivered to the glow discharge space via a second hollow capillaryrather than in conjunction with the electrolyte solution. For instance,FIG. 3 illustrates a portion of a device that includes the glowdischarge space 135 defined between the discharge end 124 of a firsthollow capillary 122 and the discharge end 134 of a second hollowcapillary 132. In general, the glow discharge space 135 can be definedwithin the distance between the discharge end 124 of the first hollowcapillary 122 and the discharge end 134 of the second hollow capillary132. For example, the distance that defines the glow discharge space 135can be from about 0.1 mm to about 5 mm. More specifically, the glowdischarge space 135 can encompass that area in which the flow that isdischarged from the first hollow capillary and the flow that isdischarged from the second hollow capillary will intersect, thusinjecting the analyte carried by the second hollow capillary into themicroplasma of the glow discharge.

The size of the glow discharge space can also be variable on a device.For instance, one or both of the discharge ends of the first hollowcapillary and the second hollow capillary can be selectively movable sothat the size and/or configuration of the glow discharge space 135 canbe varied.

At least a portion of the first and second hollow capillaries 122, 132can be in electrical communication with a power source. The power sourceis not particularly limited. For instance, the power source can be adirect current source. In other embodiments, the power source can be aradio frequency power source, a microwave frequency power source, or anyother suitable power source as is generally known in the art. A suitabledirect current power source for maintaining the glow discharge can beprovided by a Kepco (Flushing, N.Y.) Model BHA 2000-0.1M power supply.

In one embodiment, the discharge end 124 of the first hollow capillary122 can be formed of a material that is electrically conductive, such asa metal, e.g., stainless steel. This is not a requirement of the device,however. As the electrolyte solution carried within the first hollowcapillary 122 functions as an electrode during use, the discharge end124 of the first hollow capillary 122 can alternatively be formed of amaterial that is electrically insulating (such as a polymer (e.g., PEEK)glass or fused silica for example) and/or material that is electricallysemiconducting (such as silicon). In an embodiment in which thedischarge end 124 of the first hollow capillary 122 is formed of anelectrically insulating material, the first hollow capillary 122 caninclude an electrically conductive element (such as a capillary section,for instance) at a point upstream of the discharge end 124. By way ofexample, in one embodiment the first hollow capillary 122 can include anelectrically conductive element (either at the discharge end 124 orupstream) of stainless steel.

The discharge end 134 of the second hollow capillary 132 can generallybe formed of a conductive material, for instance a metal such asstainless steel, copper, etc. to form the counter electrode of thedevice.

The inside diameter of the first hollow capillary 122 and the secondhollow capillary 132 can be of a size to deliver the relatively lowvolumes of electrolyte solution and analyte utilized in the disclosedmethods. For instance, the inside diameters of the first and secondcapillaries can independently be from about 0.1 mm to about 2 mm, orfrom about 0.2 mm to about 0.5 mm in some embodiments.

During use, the power supply can be connected between an electricallyconductive element of the first hollow capillary 122 and the dischargeend 134 of the second hollow capillary 132. An electrically conductiveelement of the first hollow capillary 122 can also be in electricalcontact with the electrolyte solution that flows through the firsthollow capillary so as to place a potential difference in the range offrom about 200 volts to about 1,000 volts (direct current) across theelectrolyte solution as it exits the discharge end 124 of the firsthollow capillary 122 and the discharge end 134 of the second hollowcapillary 132.

The polarity of the system can generally depend upon the nature of theelectrolyte solution and the analyte to be examined. For instance, whilethe discharge end 134 of the second hollow capillary 132 is typicallythe powered (or input) electrode, and the electrolyte solution passesthrough the first hollow capillary 122 that is typically the outputelectrode, this is not a requirement of the device, and the oppositepolarity can be established in other embodiments.

FIG. 4 is an expanded view of the glow discharge space 135 andsurrounding components of an LS-APGD device. As shown, the first hollowcapillary 122 can carry an electrolyte solution 127 within the capillary122 toward the discharge end 124 as indicated by the directional arrow126. The second hollow capillary 132 can carry an aerosol that carriesthe analyte 137 within the capillary 132 toward the discharge end 134 asindicated by the directional arrow 133. The LS-APGD generally caninclude a mechanism for moving the electrolytic solution and the aerosolthrough the respective capillaries so as to be expelled from thedischarge ends of the capillaries. For instance, the electrolyticsolution 127 can be expelled from the discharge end 124 of the firsthollow capillary 122 at a rate in the range of about 1 microliter perminute (μ/min) to about 5 milliliter per minute (mL/min) at atmosphericpressure. A carrier gas can deliver the analyte as an aerosol to theglow discharge space 135 at a rate in the range of from about 0.1 L/minto about 1 L/min, for instance from about 0.15 L/min to about 0.8 L/min,from about 0.2 L/min to about 0.7 L/min, or from about 0.3 L/min toabout 0.6 L/min, in some embodiments.

Flow through the hollow capillaries can be accomplished in a number ofways. For example, a high precision liquid pumping system such as a highperformance liquid chromatography (HPLC) pump can be utilized for aliquid flow such as the electrolyte solution through the first hollowcapillary 122. The inlet end of the capillary 122 can be connected influid communication with the outlet of a pump. One example of such asuitable HPLC pump is a Waters (Milford, Mass.) Model 510 HPLC pump.

In another embodiment, flow through the capillaries can be encouraged byinducing electro-osmotic flow. In this embodiment, a direct currentpower supply can place an electric potential along the length of asection of one or both of the capillaries 122, 132 that terminates inthe discharge end(s) 124, 134. An electric field thus established causespositively charged particles inside the capillary to migrate toward thedischarge end. As the positively charged particles move, they carryalong the non-charged species due to the effect of the fluid'sviscosity, and momentum carries the fluid out of the discharge end ofthe capillary.

In yet another embodiment, flow through one or both of the capillariescan be accomplished by capillary action.

In one embodiment, the same power supply that is used to move theelectrolyte solution through the capillary 122 (e.g., viaelectro-osmosis) and/or the analyte through the capillary 132 can beused to provide the power needed to maintain the microplasma that isproduced at the glow discharge 136 at atmospheric pressure at the glowdischarge space 135.

The LS-APGD device can be a glow discharge optical emission spectroscopy(GD-OES) source that operates at atmospheric pressure. As stated, one ofthe discharge electrodes of the LS-APGD device is the electrolyticsolution. The passage of electrical current (either electrons orpositive ions) across the solution/gas phase interface causes localheating and the volatilization/excitation/ionization of the analytespecies that is discharged into the glow discharge space at the secondelectrode. As with previously known LS-APGD devices, the LS-APGD canfunction so as to totally consume an aqueous electrolyte solution at aflow rate of up to about 2 mL/min, i.e., no water drips from thedischarge end 124 of the capillary 122. During use, the heat generatedby the glow discharge 136 can vaporize the electrolyte solution 127 atthe discharge end 124 of the capillary 122.

When an electrolyte solution 127 that has been electrified by theelectrically conducting element of the first hollow capillary 122 isconsumed as it emerges from the discharge end 124 of the capillary 122,a glow discharge 136 including the microplasma is created between theemerging surface of the exiting electrified electrolyte solution 127 andthe discharge end 134 counter electrode of the second hollow capillary132. The microplasma can have i-V characteristics that are within therange of conventional (abnormal and normal) glow discharges operating inthe 0.1 to 10 Torr pressure regime.

In accordance with one embodiment as illustrated in FIG. 3 and FIG. 4, asheath/cooling gas can flow around at least a portion of the firsthollow capillary 122 including the discharge end 124. For example, theterminal portion of the first hollow capillary 122 including thedischarge end 124 can be surrounded by a conduit 131 such as aconcentric cylinder. This conduit 131 can be connected to a gas supplysuch as helium, nitrogen or argon gas. The gas flow (indicatedschematically by the arrows designated by the numeral 138) around theexterior of the discharge end 124 of the capillary 122 can keep thetemperature of the discharge end 124 of the capillary from exceeding themelting temperature of the materials that form the discharge end 124 ofthe capillary 122. Additionally, the gas 138 exiting from the annularspace that is defined between the exterior of the discharge end 124 ofthe capillary 122 and the interior of the surrounding cylinder 131 isbelieved to confine the plasma of the glow discharge 136 more tightlyaround the longitudinal axis 126 of the discharge end 124 of thecapillary 122. Furthermore, the gas 138 exiting from the annular spacethat is defined between the exterior of the discharge end 124 of thecapillary 122 and the interior of the surrounding cylinder 131 isbelieved to create an environment that is especially conducive to theformation of the glow discharge 36 and can help improve the temporalstability of the plasma. When included, the gas flow rate of asurrounding gas flow 138 can generally be from about 0.5 mL/min to about2 L/min, for instance from about 1 mL/min to about 1 L/min in oneembodiment.

While the second hollow capillary 132 is shown aligned at about 180°with the longitudinal axis 126 of the discharge end 124 of the capillary122 in FIG. 3 and FIG. 4, it should be understood that this alignment isnot required and the discharge end 124 of the first hollow capillary 122and the discharge end 134 of the second hollow capillary 132 can bealigned in any desired configuration where additional room or adifferent angle is desired. For instance, a particular configurationbetween the two can be provided to accommodate the inlet of a massspectrometer.

By way of example, as shown schematically in FIG. 5A, the discharge end124 of the first hollow capillary 122 can be disposed such that thelongitudinal axis 126 of the capillary 122 at the discharge end 124 isdisposed in axial alignment with one another, as shown above, and thisaxial alignment can be in any plane. In one embodiment, an axialalignment of the capillaries 122, 132 can serve to improve containmentof the microplasma formed in the glow discharge space 135 and mayincrease retention time of an analyte within the microplasma.

FIG. 5B illustrates another arrangement between the first hollowcapillary 122 and the second hollow capillary 132. As can be seen, inthis embodiment the second hollow capillary 132 is not disposed insymmetrical axial alignment with the axis 126 of the first hollowcapillary 122. Rather, in this embodiment, the longitudinal axis of thesecond hollow capillary 132 is disposed at an angle to the axialdirection 126 of electrolyte flow from the discharge end 124 of thecapillary 122. This angle can be varied depending upon the systemparameters. For instance the angle between the longitudinal axis of thefirst hollow capillary and the longitudinal axis of the second hollowcapillary can be less than 90°, about 90°, or greater than 90°.

FIG. 6 illustrates another embodiment of an arrangement between thefirst hollow capillary 122 and the second hollow capillary 132 of anLS-APGD device. As can be seen, in this embodiment, the first hollowcapillary is surrounded by a cylinder 131 through which can flow a gas138 that can be fed to the annular space of the cylinder. The two hollowcapillaries 122, 132 are at a 40° to one another with the glow discharge136 formed in the glow discharge space 135 where the flow of theelectrolyte solution 127 that exits the discharge end 124 of the firsthollow capillary 122 and the flow of the analyte 137 that exits thedischarge end 134 of the second hollow capillary 132 intersect. Themicroplasma of the glow discharge 136 can increase the energy of theanalyte for evaluation by a suitable instrument 142. A voltagedifference can be established by use of a power supply 140 in electricalcommunication with the second hollow capillary 132 such that thedischarge end 134 of the second hollow capillary 132 is a counterelectrode and also in electrical communication with a conductive elementof the first hollow capillary 122. The electrical circuit can includestandard components as desired such as resistors 141 and the like.

In general, the electrically conducting element of the first hollowcapillary 122 can be electrically insulated from the rest of theapparatus that is disposed upstream from the electrically conductingelement. This can be accomplished for example by providing anelectrically insulating conduit that forms the portion of the capillary122 that is disposed upstream from the electrically conducting element(e.g., a metallic section). In an alternate embodiment, the first hollowcapillary 122 can be formed entirely of electrically insulating material(e.g., polyetherether ketone (PEEK)) and the electrically conductingelement can be formed as an electrically conducting probe (such as ametal wire) that enters the interior of the capillary 122 through a sidewall of the capillary.

The LS-APGD affords the option of applying a voltage across the glowdischarge space in any of four ways: (1) the electrolytic solutiongrounded acting as the anode of the circuit; (2) the electrolyticsolution powered as the anode of the circuit; (3) the electrolyticsolution grounded as the cathode of the circuit; and (4) theelectrolytic solution powered as the cathode of the circuit. Theelectrode that is at the more negative potential always serves as thecathode. For instance, in one embodiment, the electrically conductingelement of the first hollow capillary 122 can be electrically connectedto the electrical power supply so as to become the powered (or input)electrode while the discharge end 134 of the second hollow capillary 132can become the output electrode.

According to one embodiment, the LS-APGD device can be utilized inconjunction with a laser ablation device, which can provide the analyte137 that is carried in the second hollow capillary 134. FIG. 7schematically illustrates one embodiment of a laser ablation device asmay be utilized in such a system. The laser ablation system can directelectromagnetic energy in the form of a laser beam 210 from a laser to asolid sample 202 causing at least a portion of the solid sample 202 tobe ablated and forming particles, or in the case of LIBS, causing thesolid sample to be vaporized. In one embodiment, the laser can formanalyte particles of a size on the single nanogram scale. By way ofexample, a femtosecond pulsed laser, such as has been utilized in thepast in conjunction with inductively coupled plasmas can be utilized(e.g., a J100 Series femtosecond laser, available from Applied Spectra,Fremont, Calif.). There is no particular limitation on the laser of alaser ablation system, provided it delivers suitable energy to ablatethe solid sample. For instance, in one embodiment a yttrium aluminumgarnet (YAG)-based laser, e.g., a 1064 nm neodymium:YAG (Nd:YAG) laserwith a nanosecond pulse temporal width (e.g., about 5 nanosecond pulse)can be utilized with a solid metal sample.

A carrier gas supply 216 can enter the sample chamber 208 via inlet 218such that the carrier gas passes through the chamber 208 and picks upthe analyte to form an aerosol prior to exit via outlet 220. The carriergas can generally include any as is generally known in the art such as,without limitation, helium, hydrogen, argon, nitrogen, air, etc. Theoutlet can be the second hollow capillary that discharges into the glowdischarge space of the LS-APGD device or may be a different line, but influid communication with the second hollow capillary of the LS-APGD. Ingeneral, the carrier gas can be carried at a flow rate of about 1 literper minute or less, for instance from about 1 milliliter per minute(mL/min) to about 500 mL/min, from about 10 mL/min to about 400 mL/min,or from about 50 mL/min to about 350 mL/min, in some embodiments.

Optionally, and as illustrated in the embodiment of FIG. 7, the laserablation system can include a reflector 212 for directing the laser beam210 and a lens 214 that can focus the beam 210 and direct theelectromagnetic energy of the laser to the sample 202, for instance witha focused target size of from about 10 to about 1 millimeter.

The sample 202 can be held on a stage 204 that in one embodiment can bemobile, for instance by use of a stepper motor 206, which can cause thestage 204 to move as desired in one, two, or three directions, forinstance in steps of about 1 millimeter or less.

The LS-APGD glow discharge device can be provided as a component of asystem that can include a laser ablation device and an analysisinstrument. For instance, one embodiment of a system is illustrated inFIG. 8. As shown, the system can include a laser ablation device 301, anLS-APGD device 302, and an analysis instrument 303. The LS-APGD devicecan be a simple and compact source capable of processing an analyte inan aerosol obtained by use of the laser ablation device 301. Analyte ofthe aerosol generated by laser ablation device 301 can be vaporized,excited, and/or ionized by the LS-APGD device 302 and analyzed by theinstrument(s) 303. The laser ablated analyte is transported by means ofa carrier gas 316 flowing to the second hollow capillary 332. Forinstance, the carrier gas can flow at about 1 ml/min throughpolyurethane tubing connected to the second hollow capillary 332, whichis conductive at the discharge end, e.g., a stainless steel capillary.The LS-APGD device produces a microplasma between the electrolytesolution (e.g., a 5% nitric acid solution) delivered by a device 320such as a syringe pump through the first hollow capillary 322.Optionally, the electrolyte solution can be delivered in conjunctionwith a sheath gas flow 317 as discussed above. The electricallyconductive end of the second hollow capillary 332 functions as thecounter electrode, and can be held at about 600 VDC in one embodiment.The first hollow capillary 322 that delivers the electrolyte solutioncan be, e.g., a fused silica capillary surrounded by an outer stainlesssteel capillary so as to provide a sheath/cooling gas 317 around thefirst hollow capillary 322. The sheath/cooling gas flow rate can beabout 1 ml/min.

The voltage between the electrolyte solution and the counter electrodecan be provided by a power supply 340, and the circuit can includevarious control devices, e.g., a 10 kilo-ohm, 225 W resistor that canprovide ballast between the two electrodes. The geometry of the twohollow capillaries 322, 332 allows the analyte-containing carrier gasaerosol to be injected directly into the plasma for interaction andenergy transfer from the plasma to the analyte. The system can includean analysis device such as a spectrograph coupled to an intensifiedcharge-coupled device 304 and can include collection optics 305 directedat the glow discharge space 335 and optionally also at the laserablation device to affect simultaneous LIBS measurements.

In one embodiment, the electrolyte flow can contain a referencematerial, for instance a known amount of the analyte or another materialthat can function as an internal standard during use of thedevice/system.

Beneficially, it has been found that the microplasma formed by theLS-APGD device can possess sufficient thermal energy to vaporize widelyemployed nanosecond, infrared laser-produced particles, which aregenerally larger than those formed via femtosecond ablation. Beyondvaporization, it has also been found that electron collisions in theglow discharge space can be energetic enough to result in opticalemission and/or ionization that can then be used for the quantitativedetermination of analytes in the microplasma volume. The LS-APGD sourcealso offers the possibility of shortening the distance between thesampling chamber and the LS-APGD source, which can increase transportefficiency, as well as independent optimization of the sampling and thevaporization/excitation/ionization processes.

One or more instruments can be used to analyze the excited state speciesof the glow discharge. By way of example, in one embodiment, a lightdirecting element can be disposed to direct the electromagneticradiation from the glow discharge that forms in the glow discharge spaceto a suitable analyzing instrument. A suitable light directing elementcan include a fiber optic light guide. In one embodiment, one end of a 3mm core diameter liquid light guide (available from Edmund IndustrialOptics, Barington, N.J.) can be disposed so as to sample opticalemission from the plasma of the glow discharge formed in the glowdischarge space. The opposite end of the light guide can be coupled toan instrument for analyzing electromagnetic radiation that emanates fromthe glow discharge. A suitable such analyzing instrument can include amonochromator or a polychromator. Specifically, an end of the lightguide can be coupled to the entrance slit of an optical spectrometersuch as a Digikrom Model 240 monochromator (CVI Laser Corp.,Albuquerque, N. Mex.) 0.24 m Czerny-Turner spectrometer equipped with a2400 groove/mm holographic grating for optical analysis and monitoringof the emission from the sample. The control interface of themonochromator can be used to adjust the scanning range, slit width,spectral calibration, and wavelength selection of the monochromator, asis known.

An analysis instrument can be used in conjunction with otherinstruments, as is generally known in the art. For instance, aphoto-multiplier tube (e.g., from Hamamatsu, Bridgewater, N.J. Model)can be disposed to detect the optical emission signals in the glowdischarge. An analog current meter can be connected to thephoto-multiplier tube and can convert the optical emission signals intovoltage signals. A computer can be employed to record the output of thecurrent meter e.g., via a National Instruments (Austin, Tex.) NB-MIO-16×interface board. An X-Y recorder-type program within the NationalInstruments LabView 2 software environment can be used to record thedata. The obtained digital data can be processed and managed as desired.

A mass spectrometer is another instrument that can be used to analyzeconstituents of the glow discharge. A commercial inductively-coupledplasma mass spectrometer such as a Model ELAN 6100 instrument availablefrom Perkin-Elmer/Sciex, Ontario, Canada, can be disposed near theplasma of the glow discharge in a conventional manner for analyzing ionsthat emanate from the glow discharge. In another embodiment, a ThermoScientific Exactive™ orbitrap mass spectrometer can be utilized.

In one possible implementation, analyte species are ionized throughcollision with electrons or excited state atoms or through chargeexchange with other ionic species, with the products sampled via the ionoptics that extracts the ionic species from the plasma of the glowdischarge and directs them to a quadrupole mass analyzer. The masses ofthe molecules and atoms constituting the analyte flow are analyzed by amass spectrometer such as a quadrupole mass filter (or another type ofmass analyzer) that is configured to allow ions with a given mass/chargeratio to reach a detector. As is conventional, a turbo molecular pumpcan be employed to maintain the mass spectrometer under reducedpressure.

The disclosed LS-APGD can be beneficially utilized for characterizationof materials, including aerosols containing micro- and nano-particlesusing direct solid sampling, for instance via laser ablation. Thedevices are small, easily built, and easily adaptable as an energysource for analysis methods such as elemental and isotopic massspectrometry. The LS-APGD has very specific advantages over existingmethods which use high power, large volume plasma. Current plasma-basedsources rely on powers of up to 2 kW and support gases of up to 16liters per minute. The present approach involves a much smallerfootprint, lower power and gas flow. The size is also more compatiblewith efficient particle introduction.

The LS-APGD ionization source has been shown previously to be a lowcost, compact, and efficient alternative to ICP for the ionization ofanalytes carried in an electrolytic solution. The analyte deliverymethod of the disclosed devices can offer further improvements to theanalysis systems. Simplicity in design, low liquid and gas consumption,and no liquid waste are all positive attributes that can characterizethis method of sample processing for elemental analysis. The presentlydisclosed devices are capable of processing solid sample particles onthe order of about 100 nm or smaller, e.g., generated by laser ablationand delivered by carrier gas as an aerosol.

The present disclosure may be better understood with reference to theexamples provided below.

Example 1

Aerosol from bulk solid was generated using an Applied Spectra (Fremont,Calif.) model J100 series femtosecond laser ablation system. Theytterbium-doped laser crystal is directly pumped by laser diodes and,after frequency tripling, delivers 343 nm wavelength light at a spotsize of about 80 μm. Each laser pulse is 300-400 femtoseconds induration with about 110 μJ of energy. The laser repetition rate can beadjusted from 1 Hz to 10 kHz. Results obtained were acquired at 100 Hz.Lead and copper aerosols were generated while moving the sample stage at10 μm/s, while the depth-profile of a one-cent coin required astationary sample stage. Sample particles were transported by means ofargon gas flowing at 1 L min⁻¹ from a 400 cm³ ablation chamber to theion source through 25 m of polyurethane tubing having a 6 mm o.d. and 4mm i.d. This length of tubing was necessary to transport the laserablation aerosol from the laser to the mass spectrometer.

Introduction of the aerosol into the plasma of the LS-APGD utilized ahollow counter-electrode (1.6 mm o.d., 1.0 mm i.d.) connected to thecarrier gas tubing with a series of stepped down Swagelok (Solon, Ohio)connections. In addition, the geometry of the electrodes relative toeach other and relative to the sampling capillary of the massspectrometer was altered from previous devices due to the change in theaerodynamic flow brought about by the carrier gas introduction. Theconfiguration used can be seen in FIG. 6.

The cathode consisted of a fused silica capillary (125 μm o.d., 75 μmi.d.) that delivered the electrolyte solution (5% HNO₃) required tosustain the plasma, via syringe pump (Fusion 100T, Chemyx, StaffordTex.) at a flow rate of 10 μl min⁻¹, surrounded by a stainless steelcapillary (700 μm o.d., 500 μm i.d.) through which flowed Arsheath/cooling gas at a rate of ˜1 L min⁻¹. The plasma was generatedbetween the grounded solution (cathode) and a counter electrode (anode)with a constant discharge current of 18 mA. Power to the counterelectrode was provided by a Glassman (High Bridge, N.J.) model EH, 100mA, 1000V power supply through a 10 kΩ, 225 W ballast resistor. Theaddition of the argon carrier gas brought about significantly moreplasma instability that was remedied to some extent by increasing theelectrolyte flow and discharge current, however the plasma instabilityremained an issue and will likely require additional work before fulloptimization is realized. The distance from the electrodes to thecapillary inlet of the mass spectrometer was about 0.5 cm, with the gapbetween the respective electrodes maintained at a distance of about 1mm.

Mass analysis was handled by a Thermo Scientific Exactive™ Orbitrap MassSpectrometer in the positive ion mode. This instrument was designed foran electrospray ionization (ESI) source, and as such, was easily adaptedfor use with the LS-APGD ionization source, requiring only the removalof the ESI housing. The mass spectrometer was operated using in-sourcecollision induced dissociation (CID) at an energy of 52 eV. This is avalue similar to what is used with the ESI source and was utilized inthese experiments because it generally produced higher ion intensitiesin the mass region of interest and lower intensity water clusters. Dataanalysis was performed with Thermo Scientific XCalibur 2.1 software.

The first sample was a square of oxygen-free hard copper from a UHVconflat flange gasket. The second was a disc of lead (Pb) solder formedby melting several drops from a wire spool onto a flat, cool surface.The last sample was a one-cent U.S. coin. No further preparation wasperformed for any of the samples.

In order to clearly discern the response of the mass spectrometer to thesample ions relative to the background signal generated by the carriergas, data acquisition was started before or at the same time as thelaser ablation. The plasma as well as the carrier gas was in continuousoperation. The 25 m of tubing delivering the ablated particles to theLS-APGD ionization source resulted in a delay time of about 20 secondsbetween the laser firing and mass spectrometer response. FIG. 9 showsthe smoothed (15-pt. boxcar) ion chromatograms of Cu and C₄H₇ for thefemtosecond laser ablation of the Cu target at a repetition rate of 100Hz and a laser raster speed of 10 μm s⁻¹. The C₄H₇ ion at mass 55.055amu is one of several common hydrocarbon contaminants in massspectrometry. It serves, in these experiments, as the baseline massspectrometer response to the LS-APGD/laser ablation system. In additionto the C₄H₇ ion, many other common hydrocarbon ions such as thetropyllium ion (m/z 95) and fragments associated withpolydimethylsiloxane (PDMS) were found at m/z 73, 147, 207, 209. TheLS-APGD was operated at a discharge current of 18 mA, and an electrolyteflow rate of 10 μl min⁻¹.

The laser was activated and deactivated at approximately 30 seconds andat 2 minutes 45 seconds respectively. The instrument response delay wasprimarily a function of the particle transport tubing. Twenty-fivemeters of tubing with a 4 mm inner diameter has a volume of ˜0.3 L. Witha carrier gas flow rate of 1 L min⁻¹, the calculated transport time isabout 20 s, which corresponds with experimental results. The baseline Cusignal is non-zero both before and after the experiment, with the postanalysis intensity slightly elevated relative to the pre-experimentvalue. The substantial Cu background intensity is likely due to a memoryeffect caused by previous experiments which resulted in residual Cuparticles on the surface of the capillary counter-electrode and/or themass spectrometer capillary interface. After laser deactivation, washouttime was dependent not only on transport tubing length, but also onlaser ablation chamber volume. The sample particles that had beenablated were distributed throughout the volume and required time toenter the transport tubing resulting in washout times that exceedresponse times at the initiation of the experiment. Nevertheless, theresponse of the mass spectrometer when coupled to the LS-APGD and laserablation particle source clearly demonstrated the efficacy of thecombination of these analytical components for elemental analysis.

To test the plausibility of using an LS-APGD ionization source for alaser ablation depth profiling experiment, a 1994 one-cent U.S. coin wasanalyzed. Beginning in 1982, a one-cent U.S. coin consists of a 10 μmthick pure Cu shell surrounding a Zn core. While 10 μm is far from thelower limit of depth-resolution capability of laser ablation, it isstill a useful experiment in determining if the LS-APGD source canprovide adequate ionization efficiency for particles generated at laserpowers sufficiently low to resolve the Cu cladding from the Zn core.FIG. 10 shows the results of the laser ablation depth profile of thecoin using 110 μJ pulses at a frequency of 100 Hz. The LS-APGDionization source characteristics were identical to the pure Cu shardexperiment.

As in the copper shard experiment, the carrier gas and source plasmawere in continuous operation throughout the experiment, while the laserwas activated approximately 40 seconds after the start of dataacquisition. A minor isotope of Zn was plotted because the intensitymore closely matched that of copper, demonstrating the marginallyearlier arrival of the Cu particles. The background contaminant ion,C₄H₇, is also plotted. Prior to laser activation, both the Cu andhydrocarbon background intensities are significant, and follow a similartrend, while the Zn intensity is zero. As seen with the copper shardexperiment, the intensity of the Cu ion prior to laser activationindicates a significant contamination from prior extensive experimentswith pure Cu, most likely in the form of particle accumulation on theelectrodes and/or sampling orifice. Cu and Zn signal onset occur almostsimultaneously at approximately 1 min., owing to the relatively highlaser repetition rate and thin Cu cladding. The removal rate of copperis about 200 nm pulse⁻¹ which, at the 100 Hz repetition rate used,translates into complete penetration of the Cu cladding in about 0.5 s.Aerosol diffusion over the extended transport tubing results in theextended Cu intensity before the signal drops to near pre-experimentlevels after about 30 seconds, though plasma instability results insignificant fluctuations of all ion intensities throughout theexperiment. Attenuation of ion intensity is common in depth profilingexperiments as the aspect ratio (depth/diameter) increases becauseparticles have a more difficult time escaping the crater. This effectcan be seen clearly starting within 20 seconds of Zn reaching its peakintensity. Though the trend seems to also appear in the Cu signal, it ismuch more likely that this results from depletion of Cu particles,rather than crater affects given the thin nature of the claddingrelative to the laser spot size (about 80 μm).

The results presented in FIG. 11 were acquired from a disc of rosin-coresolder formed by allowing drops of melted solder wire to fall onto aflat surface. The composition of the solder was 60% Sn, 40% Pb andunspecific trace quantities of several other elements including Cu. Thelaser and LS-APGD conditions were identical to those used for the Cushard analysis. The three major isotopes of Pb are shown (FIG. 11A), allwith mass resolution exceeding 90,000, corresponding to a full width athalf maximum value of <3 mDa for the Pb²⁰⁸ isotope. Given the inverserelationship between ion mass and resolving power for the orbitrap, thePb isotope peaks exhibited somewhat lower values than the Cu isotopes,which were acquired with M/ΔM>160,000 (FIG. 11B).

Given the open nature of the LS-APGD ionization source, it is expectedthat the oxygenated species of the analyte ions will form in significantquantities due to the aqueous nature of the electrolyte solution andpossibly the atmospheric gas entrainment around the source and at theentrance to the mass spectrometer. Atomic ions that do form at thesource must travel at least several inches, after entering the capillaryinlet of the mass spectrometer, through an environment with a shortmean-free path before reaching conditions more amenable for reactivemetal ions. The expanded mass range spectrum of the commercial solder,illustrated in FIG. 12, shows this effect clearly with extensiveoxidation products of the two main constituents, Pb and Sn.

In addition to the common hydroxide and water products, it appeared thataluminum contamination products were also formed. The source of thecontamination appeared to originate from the solder, despite the factthat it is not listed as a trace metal component of the solder. None ofthe source components were known to contain aluminum. Experiments usingsolder wire as the cathode in conjunction with a solid stainless steelcounter-electrode (i.e. no carrier gas or laser ablation particles)produced similar spectra indicating that the source of aluminum did notoriginate from the laser ablation or carrier gas delivery components.

The spectrum in FIG. 12 is somewhat reminiscent of the adductedelemental spectra of ESI MS. Just as there was not sufficient energy incollisions with the counter-current gas in ESI to remove all matrixadducts (e.g. MeOH, H₂O, OH, etc.) it appeared that there was notsufficient energy in the discharge or subsequent collisions to removeall adducts in this case. However, the LS-APGD spectra are less complexthan ESI spectra since there is no organic solvent present in thesolution to lower the aqueous surface tension as required in ESI. Thesespectra are also somewhat reminiscent of the early ICP ion trap MSspectra in which case reactions with adventitious water in the threedimensional traps created similar adducts. In the ion trap case thewater/oxygen adduct ions were significantly reduced if not eliminated bycryotrapping adventitious water in the ion trap helium atmosphere.

Example 2

An experimental system is depicted in FIG. 8 was utilized. Laserablation was performed inside a sample chamber of cylindrical shape (7cm diameter and 3 cm height) with one gas port inlet and one outlet.Helium was used as the carrier gas. The chamber was located about 1 mfrom the LS-APGD.

The counter-electrode was a nickel, 0.3 cm o.d., 0.1 cm i.d. hollowcapillary, the solution electrode (nickel, 0.16 cm o.d., 0.06 cm i.d.)housed an internal fused silica capillary (360 μm o.d., 100 μm i.d. IdexHealth and Science). Helium was introduced as the sheath/cooling gas(0.1-0.7 L min⁻¹) in between the concentric capillaries with a liquidflow (1-100 μL min⁻¹) of 5% HNO₃ being introduced through the fusedsilica capillary using a programmable syringe pump (New Era Pump SystemsInc., model NE-1000 Multi-Phaser). The distance between the twoelectrodes was kept between about 1 and 2 mm. Power was provided to thecounter electrode by a Bertan Model 915 series power supply (0-100 mA,0-1 kV, Hicksville, N.Y.) that was operated in negative polarity andconstant current mode with a 10 kΩ, 225 W ballast resistor.

The laser ablation device utilized a Nd:YAG laser with a 5-ns pulseduration operating at its fundamental wavelength of 1064 nm, at arepetition rate of 5 Hz. The laser beam was focused onto the targetperpendicular to the sample surface using a lens with a focal length of100 mm. The laser energy used for ablation was 44 mJ. The sample wasplaced on an x, y, z manual stage. After selecting the laser energybased on the APGD signal, LIBS emission signal, and signal-to-backgroundratios from Cu (324.75 and 515.33 nm) were used to set detector gatedelay, width and gain. The operation parameters were:

Laser type and wavelength: Nd:YAG 1064 nm

Pulse duration: 5 ns

Pulse energy: 44 mJ

Repetition rate: 10 Hz

Focal length of the lens: 100 mm

Spot diameter: about 100 μm

Delay time: 1 μs

Gate width: 5 μs

Gain: 1

An intensified charged-coupled device (ICCD) from Princeton Instrumentsdetected optical emissions from both plasmas (LIBS plasma and LS-APGDplasma). The ICCD was triggered from the Q-switch sync output of thelaser. An Acton SP2150i spectrograph with a 1200 grooves mm⁻¹ gratingwas used to disperse the atomic emission. The wavelength range of thespectrometer was centered at 320 nm and 515 nm. A fused silica biconvexlens (35 mm focal length, 25.4 mm diameter) was used to focus theemission onto the entrance of an optic fiber, and into the 750 μmentrance slit of the spectrograph. The same detector was used to measureemission from the LS-APGD; in this case the spectrometer was used inshutter mode because the microplasma was continuous.

A pure copper standard (Goodfellow, Oakdale, Pa., USA) was used toevaluate the LS-APGD performance. A set of copper-zinc binary alloys(Glen Spectra reference materials (Middlesex, UK) was used to assess thequantitative aspects of the LS-APGD as a potential second excitationsource for laser ablation. The alloy compositions are provided in thetable, below.

CRM Zn (%) Cu (%) Zn/Cu 1 48.5 51.5 0.943 2 44.4 55.5 0.799 5 30.5 69.50.439 7 17.5 82.5 0.212 9 10.5 89.5 0.117 10 6.2 93.8 0.066

The sample surfaces were slightly polished and then cleaned with ethanolprior to the ablation to improve the reproducibility of themeasurements. Two hundred (200) laser shots at 5 Hz were delivered toeach position on the sample, with the measured optical intensitiesintegrated over the duration of the experiment. Measurements from threedifferent locations of each of the samples were performed and the meanand standard deviation of these three measurements were used for theplots.

The mean particle size in this application could reach up to 2 μm. Theconfiguration utilizes a 180° geometry (collinear with respect to thesolution electrode and counter electrode), which was found to be anoptimal geometry for optical emission studies. In the case of LAsampling, this geometry also ensured the longest residence time in theplasma. Systematic parameter evaluations were conducted. The parametersof interest included: particle carrier gas flow rate (helium), glowdischarge current, laser energy, and the solution electrode sheath gasand liquid flow rates. The quantitative performance was assessed usingthe brass alloy reference materials, with comparison of results toLA-ICP-OES.

The OES response of the ablated material to changes to the microplasmasheath gas flow rate, the flow rate of the electrolytic solution (5%HNO₃), and the carrier gas (transporting the ablated particles into theplasma) flow rates are interrelated. The LS-APGD was initially optimizedusing a copper standard solution to ensure proper operation prior tointroducing ablated particles. FIG. 13 shows the Cu (I) 324.75 nmresponse for ablated particles introduced into the plasma via the heliumcarrier gas, operated from 0.15 to 0.62 L min⁻¹. The maximum copperresponse was found using a helium carrier gas flow of 0.30 L min⁻¹. Atflow rates <0.3 L min⁻¹, a large drop in intensity was measured, likelydue to poor transport from the sample chamber to the LS-APGD. At flowrates 0.35 L min⁻¹, the drop in intensity was more gradual than at lowflow rates, suggesting that particles were reaching the LS-APGD, butpassing through the plasma faster than they could be vaporized/excited.The velocity of the carrier gas under the optimum 0.3 L min⁻¹ flow ratesuggests a plasma residence time of about 1 ms.

The electrolyte solution (5% HNO₃) through the glass capillary wasdelivered at rates of 1-100 μL min⁻¹ while measuring the emission of theablated copper particles introduced by the carrier gas flow rate of 0.30L min⁻¹. In this implementation of the LS-APGD, the solution flow ratecontrolled the delivery of current carrying charge to the surface of theliquid electrode as well as the solvent loading into the plasma.

Measurements revealed that at flow rates of 1-25 μL min⁻¹ the copperresponse was consistent, but as the flow rate was increased from 25 to100 μL min⁻¹ the intensity began to drop and the precision wascompromised. At this discharge current (60 mA), the higher solution flowrates consumed more of the total plasma energy and affect the solutionevaporation and gas phase excitation processes.

The sheath gas flow surrounding the liquid capillary of the microplasmaserved to provide stability, particularly at low liquid flow rates.Using a flow rate of 10 μL min⁻¹ (5% HNO₃) and a particle carrier gasflow of 0.30 L min⁻¹, the flow rate of the sheath helium gas was variedfrom 0.13 to 0.57 L min⁻¹. The maximum copper response was measuredusing flow rates between 0.13 and 0.23 L min⁻¹, with a gradual drop incopper intensity found at flow rates 0.27 L min⁻¹. As the sheath gas wasincreased above 0.3 L min⁻¹ the path of the particles were likelyimpeded due to turbulence in the counter flow of helium gas. An attemptto operate the LS-APGD source with no sheath gas resulted in overheatingand melting of the capillary and solution electrode tip. The optimizedconditions of 0.30 L min⁻¹ helium carrier gas flow, 0.20 L min⁻¹ heliumsheath gas flow, and a liquid flow rate of 10 μL min⁻¹ 5% HNO₃ wereemployed throughout the remainder of these studies.

The analyte excitation was found to be most efficient at relativelyhigher currents (35 mA). FIG. 14 displays the Cu (I) response as afunction of glow discharge current for particles introduced fromablation of the copper target. It takes mA of current to excite theablated copper particles, with essentially no signal detected below 45mA. The particles ablated from the 1064 nm nanosecond pulsed laser wererelatively large, and thus it is reasonable to suggest that more energywas required to vaporize and excite them as compared to experimentswhere the plasma was operated at a current of 10 mA. The remainder ofthe experiment was carried out using a glow discharge current of 60 mA.

The final operational parameter studied was the incident laser energy.The incident laser energy affects two processes; the size of the ablatedparticles and the total ablated mass per unit time. FIG. 15 shows alinear fit to the Cu (I) intensity as a function of laser energy. Whilethe response looks to be fairly proportional to the input power, thereis some indication that there may be an onset of saturation of theprocess. Overall, it is believed that the plasma was not overloaded withablated particles in these experiments. A laser energy of 44 mJ waschosen for the remainder of the example.

FIG. 16 represents a typical optical emission spectrum collected from acopper standard ablated into the LS-APGD using the optimized conditions.The copper emission at wavelengths of 324.75, 327.39, 510.55, 515.33,and 521.82 nm is clearly discernible from the background obtained whenparticles are not ablated into the LS-APGD source. The large band, witha maximum at 337.13 nm corresponds to emission of the molecular bandN₂C₃

-B₃

(0,0), most likely from the nitrogen in air and/or perhaps the nitricacid solution used to sustain the plasma.

The set of alloys was ablated and analyzed by both LA-LS-APGD and LIBS.These alloys have been studied extensively using LA-ICP-MS in thislaboratory, providing a benchmark for understanding the laser ablatedLS-APGD and LIBS measurements. Analysis by LIBS provided a non-linearitycalibration response, with a good response to Zn-containing smallerparticles, and poor sensitivity to larger Cu particles. When Cu was usedas an internal standard, a linear calibration response was measured forthe Zn/Cu alloy standards. FIG. 17 is illustrative of the spectrameasured from the LS-APGD in the ablation of brass alloys, in this casealloy #1 (51.5% Cu/48.5% Zn). The spectra derived from the LS-APGDexcitation source are similar in composition (relative to the species'contributions) to direct LIBS analysis of brass samples. The spectrareflect nucleation and condensation from the vapor, producing smallerparticles which are Zn-rich due to its much lower vaporizationtemperature in comparison to Cu. Copper is more difficult to vaporizeand instead larger copper particles are ejected from the melted surfaceof the brass alloy and transported into the plasma region. The separaterole of fractionation during laser ablation versus that which may occurin the LS-APGD has not been addressed.

With low repetition rate sample introduction, there is a temporalcomponent to the measured response, reflective of the particulatetransport to the excitation/ionization source. FIG. 18 shows thetransient response for two Cu (I) emission lines and one correspondingto Zn (I) for the laser ablated LS-APGD microplasma. These data show howintensities increase during the time the laser is on, reaching maximumvalues at the same time, with a slow decrease when the laser is switchedoff. Particles were found to take about 3.5 seconds to reach the LS-APGDsource from the sampling chamber. The fact that these two elementalspecies were well correlated suggests that either the plasma wasefficient at vaporizing the various particle sizes or that only thesmaller fraction of particles was transported.

While the optical spectra of the LA particles in the microplasma can betreated as a transient on a continuous background, the LIBS spectra mustbe time-gated from background plasma continua on a single-pulse basis.FIG. 19 shows a typical spectrum obtained from direct observation of theLIBS emission for the same brass alloy as FIG. 17 using the procedureparameters of Table 1. All relevant emission lines in this region for Cuand Zn are observed and, in contrast to the spectra obtained from laserablated LS-APGD emission, no N₂ band was observed (e.g., FIG. 16A). Incomparison of FIG. 19 and FIG. 17, differences in the excitationconditions between the LS-APGD and LIBS plasmas as the higher-lying Zn(I) lines are more prevalent than the Cu (I) resonant transitions. Ingeneral, the S/B for the Zn (I) lines from the microplasma are ˜2× thoseof the LIBS source, with Cu (I) being quite comparable.

The quantitative characteristics of the laser ablated LS-APGD and LIBSmethods were compared using the brass alloys listed in the table above.FIG. 20 shows the integrated emission intensities (Cu (I) and Zn (I))for each brass alloy for both approaches. The Zn response curve slopesfrom LA-LS-APGD and LIBS and are similar, with values of 7819 and 8566,respectively. The Zn response from the microplasma was a factor of 4-5×greater than from the LIBS measurements alone; this was not the case forthe Cu emission line as the LIBS signal was more intense than from thelaser ablated LS-APGD plasma. Under these conditions, the LIBSmeasurements did not detect changes in Cu concentrations via the Cu (I)intensities, whereas the laser ablated LS-APGD measurements show aslight increase in response as the copper concentration was increased.The differences in the intensities for the two elements reflect adifference in the excitation conditions (temperatures) between the twoplasma environments. Alternatively, this may be an indication that themicroplasma is less effective in vaporizing the larger particles thanthe LIBS plasma. Even so, concentration-dependent responses are observedin the case of microplasma sampling.

As a final measure of performance, the ability to obtain qualitativeresponse curves from the brass alloys was evaluated. Data in FIG. 20 forCu and Zn intensities followed similar behavior as previously reportedusing LA-ICP-MS. As shown in FIG. 21, the Zn (I)/Cu (I) ratio was foundto be linear with the elemental composition ratios for both techniques,with good correlation coefficients (R²).

While a presently preferred embodiment of the invention has beendescribed using specific terms, such description is for illustrativepurposes only, and it is to be understood that changes and variationsmay be made without departing from the spirit or scope of the followingclaims.

What is claimed is:
 1. A liquid sampling, atmospheric pressure, glowdischarge (LS-APGD) device comprising: a first hollow tube comprising afirst inlet, a first discharge and a conductive element; a second hollowtube comprising a second inlet and a second discharge; a glow dischargespace in fluid communication with and downstream of the first dischargeand in fluid communication with and downstream of the second discharge,the glow discharge space being external to the first hollow tube and thesecond hollow tube; and a counter electrode disposed at a distance fromthe first discharge, a terminal portion of the second hollow tubecomprising the counter electrode, the counter electrode being inelectrical communication with the conductive element.
 2. The LS-APGDdevice of claim 1, further comprising a liquid source, the first inletbeing in fluid communication with and downstream of the liquid source.3. The LS-APGD device of claim 2, further comprising a pump in fluidcommunication with the liquid source.
 4. The LS-APGD device of claim 1,further comprising a gas source, the second inlet being in fluidcommunication with and downstream of the gas source.
 5. The LS-APGDdevice of claim 1, wherein the first discharge comprises the conductiveelement.
 6. The LS-APGD device of claim 1, wherein the first dischargecomprises a non-conductive material, a metal, a polymer, a glass, fusedsilica, or silicon.
 7. The LS-APGD device of claim 1, further comprisinga sample chamber in fluid communication with and upstream of the secondinlet.
 8. The LS-APGD device of claim 6, further comprising a laserablation device in communication with the sample chamber.
 9. The LS-APGDdevice of claim 1, further comprising an analysis instrument incommunication with the glow discharge space.
 10. The LS-APGD device ofclaim 1, further comprising a power source in electrical communicationwith the counter electrode and the conductive element.
 11. The LS-APGDdevice of claim 1, further comprising a conduit surrounding the firstdischarge, the conduit defining an annular space surrounding the firstdischarge.
 12. The LS-APGD device of claim 1, the conductive elementextending into an annular space of the first hollow tube.
 13. A liquidsampling, atmospheric pressure, glow discharge (LS-APGD) devicecomprising: a first hollow tube comprising a first inlet, a firstdischarge and a conductive element; a second hollow tube comprising asecond inlet and a second discharge; a sample chamber in fluidcommunication with and upstream of the second inlet; a glow dischargespace in fluid communication with and downstream of the first dischargeand in fluid communication with and downstream of the second discharge,the glow discharge space being external to the first hollow tube and thesecond hollow tube; a counter electrode disposed at a distance from thefirst discharge, a terminal portion of the second hollow tube comprisingthe counter electrode, the counter electrode being in electricalcommunication with the conductive element; and a power source inelectrical communication with the counter electrode and the conductiveelement.
 14. The LS-APGD device of claim 13, further comprising ananalysis instrument in communication with the glow discharge space. 15.The LS-APGD device of claim 13, further comprising a laser ablationdevice in communication with the sample chamber.
 16. The LS-APGD deviceof claim 13, wherein the first discharge comprises the conductiveelement.
 18. The LS-APGD device of claim 13, further comprising a liquidsource in fluid communication with the first hollow tube.
 19. TheLS-APGD device of claim 13, further comprising a gas source in fluidcommunication with the second hollow tube.
 20. The LS-APGD device ofclaim 1, further comprising a conduit surrounding the first discharge,the conduit defining an annular space surrounding the discharge.
 21. Aliquid sampling, atmospheric pressure, glow discharge (LS-APGD) devicecomprising: a liquid source; a first hollow tube comprising a firstinlet, a first discharge and a conductive element, the first inlet beingin fluid communication with and downstream of the liquid source; a gassource; a sample chamber in fluid communication with and downstream ofthe gas source; a laser ablation device in communication with the samplechamber; a second hollow tube comprising a second inlet and a seconddischarge, the second inlet being in fluid communication with anddownstream of the sample chamber; a glow discharge space in fluidcommunication with and downstream of the first discharge and in fluidcommunication with and downstream of the second discharge, the glowdischarge space being external to the first hollow tube and the secondhollow tube; an analysis instrument in communication with the glowdischarge space; a counter electrode disposed at a distance from thefirst discharge, a terminal portion of the second hollow tube comprisingthe counter electrode, the counter electrode being in electricalcommunication with the conductive element; and a power source inelectrical communication with the counter electrode and the conductiveelement.